A resolver is one type of a detection device for detecting the rotary position of a rotating machine such as a motor or a generator. Resolvers are widely used as rotary position detection devices for rotary machinery used under poor conditions, due to their ability to be used in harsher environments than encoders equipped with Hall elements or phototransistors. Resolvers of this type are typically disposed in positions adjacent to exciter coils such as motors or generators arrayed within a case. For that reason, electromagnetic noise generated by the excitation current which flows to the excitation coils in motors or generators can sometimes be superimposed onto the resolver stator excitation coils or output coils so that an accurate rotary position cannot be detected.
One type of resolver is a variable reluctance resolver structured such that excitation coils and output coils are wound not on the rotor, but on the same multiple magnetic poles of the resolver stator, and multiple stator magnetic pole output coils are serially connected to obtain a single output coil output. Such variable reluctance resolvers, as shown in FIG. 10, are provided with multiple stator magnetic poles 51 on the stator 5, multiple teeth 55 on the rotor 52, excitation coil 53, output coil 54X which outputs the rotor X directional component, and the output coil 54Y which outputs the rotor Y axis component are wound on the same pole of the relevant stator magnetic poles 51.
As shown in FIG. 11, the variable reluctance resolver is used, for example, with the output coil 54X, which outputs the X directional component of the rotor 52, and the output coil 54Y, which outputs the Y directional component of the rotor 52, connected to a resolver digital circuit 30, which produces a digital output corresponding to the resolver rotational angle. In said variable reluctance resolver, the relationship between the axis multiple angle N (the cycle count for the output voltage obtained in one revolution of the rotor 52), the area P (excitation pole pairs), the area Q (output pole pairs), and the slot (output magnetic pole) number S is expressed as shown below. In Expression 1, the amount± is appropriately determined in accordance with the multiple angle N.N=(P±Q)  (Expression 1)S=2*m*P  (Expression 2)                (m is an integer)        
In order to facilitate an understanding of the invention, an explanation is given below for a conventional variable reluctance resolver using FIG. 11. For the output of the output coil 54X, which outputs the rotor's X directional component when the direction of magnetization produced by the voltage induced at the output coil 54X by the excitation coil 53 has the same direction of magnetization as the excitation coil 53, the voltage ENS induced on a selected one of the poles is as shown in Expression 3, where the A.C. voltage VP applied to the excitation coil 53 is E sin ωt. Here ω is the angular frequency, expressed as 2π. f is a frequency, and “a” and “b” are constants determined by the excitation coil 53, the output coil 54X, and the characteristics of the rotor and the stator.ENS=(a+b sin θ)×E sin ωt  (Expression 3)
When the direction of magnetization created by the voltage induced in the output coil 54X by the excitation coil 53 differs in direction from the excitation coil 53 magnetization, the voltage ENN induced on a selected single pole is expressed by Expression 4.ENN=(−a+b sin θ)×E sin ωt  (Expression 4)
The relationship between the excitation coil 53 wound on a freely selected pole and the output coil 54X is as shown below. That is to say, the excitation coil 53 is wound in such a way that magnetic poles N and S occur alternately, one pole at a time, and the output coil 54X is wound in such a way that the N pole and S pole occur as a bipole combination. In other words, when a first pole excitation coil 53 is an N pole, the first and second poles of the output coil 54X will be N poles, and the third and fourth poles will be S poles, which will be thereafter repeated. The same is true of the output coil 54Y.
When the first and second poles of an output coil wound in this structure are serially connected, the voltage generated by poles 1 and 2, V12, changes from Expressions 3 and 4 to Expression 5.V12=(a+b sin θ)×E sin ωt+(−a+b sin θ)×E sin ωt  (Expression 5)
Similarly, the voltage V34 in Expression 6 is generated by poles 3 and 4.V34=−(a+b sin θ)×E sin ωt−(−a+b sin θ)×E sin ωt  (Expression 6)
Summarizing Expressions 5 and 6, the constant “a” term is canceled when adjacent poles are serially connected, and the voltages V12 and V34 as shown in Expression 7 are obtained.V12=2b sin θ×E sin ωt=−V34  (Expression 7)
Therefore in the case of a multiple of 2 poles, the constant “a” cancels when all pole output coils are serially connected, and the output coil 54X output voltage VS is given by Expression 8. Here, KK is a constant determined by the constant “b” (“B”) and the number of poles; with a number of poles NN. This is shown in Expression 9.VS=KK sin θ×E sin ωt  (Expression 8)KK=NN×B  (Expression 9)
Therefore, as shown in Expression 10, an output is obtained which corresponds to the change in the rotor rotational angle θ.VS=NN×B×sin θ×E sin ωt  (Expression 10)
Similarly, an output voltage for the output coil 54Y is obtained, each output is applied to the resolver digital circuit 30, and an angle is measured.
As described above, the variable reluctance resolver obtained by winding an excitation coil and an output coil around the same pole of the stator, and connecting the output coils of multiple poles serially as the output of one output coil is used in both motors and generators, etc. When a magnetic field is applied from outside the resolver, there are frequent cases in which a magnetic flux caused by the external magnetic field mixes in with the aforementioned stator of the variable reluctance resolver. The external magnetic flux induces a voltage on each of the variable reluctance resolver stator coils; each of these is respectively added, generating an induced voltage on the output coil, affecting the output VS shown in Expression 10 and degrading the accuracy of the variable reluctance resolver.
Below is explained the effect of the external magnetic flux on the variable reluctance resolver output coil using FIGS. 12 through 14. In the variable reluctance resolver shown in FIG. 12, the excitation voltage causes a voltage of same polarity S to be induced on the output coil (not shown) wound on stator magnetic poles 510 through 515 (referred to as the G1 output coil); another output coil (not shown) is wound on stator magnetic poles 516 through 521 (referred to as the G2 output coil) in order to induce a voltage having the same polarity N, different from the previous polarity S. Next, explained is the case in which an external magnetic flux φ is introduced from the axial X0 direction to the stator 5 within the variable reluctance resolver. The axis Y0 is the axis perpendicular to the axis X0.
As shown in FIG. 12, a voltage of the same polarity S is induced on the G1 output coil by the excitation voltage, and each output coil is wound in such a way that the same polarity N, different from the previous polarity S, is induced on the G2 output coil. When an external magnetic flux φ is mixed in from the axial X0 direction, the directions of the external magnetic flux φ operating on the left and right sides of the axis Y0 (the G1 output coil and G2 output coil) are mutually opposing directions. For G2, an external magnetic flux φ mixes in from the outer side of the stator 5; for G1 an external magnetic flux φ mixes in on the inside of the stator 5, which is to say the tip of each stator magnetic pole. As a result, the G2 output coil is wound in such a way as to have the opposite polarity to the G1 output coil polarity, but the voltage induced on the G1 and G2 output coils have the same polarity (N) and N, which are respectively added and output. In other words, all of the output wires are affected by external magnetic fluxes.
The effect of external magnetic flux on the output coil in a variable reluctance resolver having an output coil with a number of fixed magnetic pole teeth different from FIG. 12, in which, as in FIG. 13, the axial multiple angle is 5 and the number of fixed magnetic poles is 20, is as shown below. In the variable reluctance resolver of FIG. 13, an output coil (not shown) is wound in such a way that a same polarity S voltage is induced on the output coil (called the group A output coil) wound around a pair of 2 adjacent stator magnetic poles (for example stator magnetic poles 611 and 612) and a voltage of polarity N different from the group A output coil polarity S is induced on the output coil (called the group B output coil) wound around a group of 2 stator magnetic poles adjacent to group A. As shown below, the group A output coil and group B output coil are alternately wound, with 2 adjacent stator magnetic poles forming one group.
The effect imparted on an output coil by the external magnetic flux φ when an external magnetic flux φ is mixed into the stator 5 from the axis X0 direction (the horizontal direction between the stator magnetic poles 624, 625) in the variable reluctance resolver is now explained in connection with FIG. 13. Axis Y0 is perpendicular to the axis X0. As shown in FIG. 13, a same polarity S voltage is induced in each of the group A output coils by the excitation voltage applied to the excitation coil, and a voltage of same polarity N, different from the polarity S, is induced in each of the group B output coils. However, when the external magnetic flux φ is introduced from the axis X0 direction, the external magnetic flux φ directions on the left and right side of the axis Y0 are mutually opposing directions (on the left side of the axis Y0, the external magnetic flux φ mixes in from the outside of the stator 5, and on the right side of the axis Y0, the external magnetic flux φ mixes in from the inside of the stator 5, which is to say from the tip of each stator magnetic pole).
Therefore, the output coils are wound in such a way that the voltages induced in each output coil by the voltages applied to the excitation coil have the same polarity for each Group A output coil, and have the same polarity for each output coil in Group B within the applicable group, but for the induced voltage with respect to the external magnetic flux φ the direction of external magnetic flux with respect to the output coil is different between the left and right sides of the axis Y0. In other words, the polarity of the induced voltage in the output coil wound on the right side of the axis Y0 reverses and becomes the opposite polarity (shown respectively by the letters (S) and (N)). As a result, the induced voltages generated by the external magnetic flux φ cancel one another out between the adjacent output coils on the same side as axis Y0, or between the output coils positioned to mutually face the axis X0 (for example, between the output coils 629 and 620 as indicated by the dotted line K).
Meanwhile, the case when the external magnetic flux φ is introduced into the stator 5 from an angle and axis X0 direction (the horizontal direction between the stator magnetic poles 625, 626) that is different from the angle shown in FIG. 13 is shown in FIG. 14. That is, the induced voltage generated by the external magnetic flux φ is not canceled between the adjacent output coils on the same side as axis Y0, or between the output coils positioned to mutually face the X0 axis, whereas the voltage induced between the group A output coils (for example, between output coils 628 and 629) do mutually cancel one another. However, the external magnetic flux φ and the stator magnetic pole angle differ in said mutually canceling output coils (for example, between output coils 628 and 629), and therefore the proportion of external magnetic flux φ mixing differs. As a result, the induced voltage remains within each group without being canceled.
The winding is such that the voltages produced by excitation voltages applied to excitation coils have respectively opposite N and S polarities in output coils 610, 621 and output coils 611, 620, but the directions of the external magnetic flux φ which works on output coils 610, 621 and output coils 611, 620 are respectively reversed, since the external magnetic flux φ direction is reversed. As a result, the output coils 610, 621 and 611, 620 have the opposite polarities N, S, but the voltages excited in both coils respectively each take on the same polarity N (N); each is respectively added together, and the output coils 610, 611, 620, 621 (the output coils outlined by reference numeral K1) directly receive the effect of the external magnetic flux.
In other words, as shown in FIG. 13, voltages of the same polarity S are induced on a selected group of output coils, taking 2 adjacent stator magnetic poles as one group, and in the variable reluctance resolver, in which output coils are alternately wound in such a way that voltage[s] of the same polarity N, different from said polarity S, are induced in the output coils in another group adjacent to said selected group, the degree of effect therefrom is different according to the direction from which the external magnetic flux φ mixes.
Various magnetic shields for magnetic shield placement on various parts are disclosed in order to resolve the negative effects arising from external magnetic fields received by said resolver. For example, there is the invention disclosed in Laid Open Patent Publication 2001-191931, in which the motor stator and the resolver stator are affixed within the same housing, and a hollow, disk-shaped magnetic shield part is disposed between said motor stator and said resolver stator.
However, said magnetic shield requires a shield part with low magnetic resistance. Such shield materials are expensive, and shielding the variable reluctance resolver causes the device to increase in size. Furthermore, it is necessary to know the direction of the external magnetic flux and to place the magnetic shield where it will be effective, but there are many cases in which it is difficult to determine said effective position. At the same time, if a magnetic shield is not provided, the effect of external magnetic flux will be received by all the output coils in output coils such as shown in FIG. 12, and, in output coils such as shown in FIG. 13, and problems will arise such as different output coils receiving the effect due to the direction in which the external magnetic flux mixes.